1. Technical Field
The present invention relates to a nonvolatile semiconductor memory device and a manufacturing method thereof. The present invention preferably applied to a nonvolatile semiconductor memory having a silicon-on-insulator (SOI) transistor and a floating gate that is formed on the back side of the SOI transistor.
2. Related Art
Some nonvolatile semiconductor memory devices have a control gate formed on the channel region through a floating gate for the purpose of electrical data writing/erasing. When writing of data is performed in the nonvolatile semiconductor memory device, a high voltage is applied to the control gate and electrons are injected into the floating gate through a tunnel effect. When data erasing is performed in the nonvolatile semiconductor memory device, a high voltage is applied to a substrate side and the electrons that are accumulated in the floating gate are drawn out to the substrate side through the tunnel effect.
Separation by Bonding Si Islands (SBSI) for LSI Application (T. Sakai et al.; Meeting Abstract of Second International GiGe Technology and Device Meeting; p. 230-231; May, 2004) is an example of related art. The example disclosed a method of forming a SOI transistor at a reduced cost by forming a SOI layer on a bulk substrate. According to the method described in the example, first, a Si/SiGe layer is formed on a Si substrate, and a cavity is formed between a Si substrate and the Si layer by exclusively removing the SiGe layer by utilizing a difference in the selectivity of Si and SiGe. Second, a SiO2 layer is formed so as to be buried between the Si substrate and the Si layer by performing a thermal oxidation of the Si layer exposed in the cavity. In this way, a buried oxide (BOX) layer is formed between the Si substrate and the Si layer.
However, it is difficult to make the gate insulating film between the control gate electrode and the channel region thinner in such nonvolatile semiconductor memory device because the floating gate that is surrounded by an insulating film is formed on the channel region. Accordingly, a threshold value at the time of data reading in the nonvolatile semiconductor memory device increases and this decreases an operation capacity. This causes a problem that device cannot accept a high-speed data reading at a low voltage level. Furthermore, the driving voltage of a sense amplifier also increases and this hampers an effort to lower the driving voltage of an integrated circuit in which the nonvolatile semiconductor memory device is included.
Meanwhile, as studies to lower the driving voltage of a logic circuit adopting a fully depleted SOI transistor advances, the circuit that can be driven at the driving voltage smaller than 0.5 V has been developed and a low-power LSI has been already realized.